The present invention relates generally to a prealloyed catalyst and the use of such a catalyst in manufacturing silicon carbide whiskers. More particularly, the present invention relates to a prealloyed catalyst containing a sufficiently high percentage of carbon, silicon, or carbon and silicon to allow growth of silicon carbide whiskers to commence immediately upon reaching the growth temperature. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Silicon carbide whiskers are valued for their needle-like single crystal structure which leads to such excellent properties as high strength, high elastic modulus, heat resistance, chemical stability, and so on. The whiskers have been used as a composite reinforcing material for metals, plastics, and ceramics. The most desirable whiskers are .beta. silicon carbide single crystals which have a high length to diameter ratio.
Various methods of using a catalyst to promote the growth of silicon carbide whiskers have been proposed. In U.S. Pat. No. 4,500,504 issued to Yamamoto, a silica gel with 6 to 25% by weight metal catalyst was mixed with furnace carbon black. The mixture then was placed in a non-oxidative atmosphere at a temperature of 1300.degree. to 1700.degree. C. to produce silicon carbide whiskers. The metal catalyst was selected from the group of iron, nickel, and cobalt. However, the metal was used as is, i.e., with no pretreatment. In U.S. Pat. No. 3,622,272 issued to Shyne et al., the use of powdered metals such as iron, manganese, nickel, aluminum, and stainless steel was disclosed for its role as a surface nucleation site. The powdered metal coatings were applied to a substrate such as graphite for the growing of silicon carbide whiskers. The only pretreatment disclosed for the metal powders was to suspend them in a liquid carrier vehicle such that the metal powders could be applied to the surface of the growth substrate.
In the article "Growth of SiC Whiskers in the System SiO.sub.2 --C--H.sub.2 Nucleated by Iron," authored by J. A. Bootsma et al., which appeared in J. Cryst. Growth 11, 297-309 (1971), the nucleation phenomenon of growing silicon carbide whiskers using iron particles as nucleating agents was studied. It was found that the iron particles first took up silicon and carbon from the vapor when the furnace temperature reached 1200.degree. C. After sufficient uptake, an Fe--Si--C alloy droplet was obtained from which the silicon carbide whisker grew. However, again there was no suggestion of pretreating the iron particle before the furnace was heated up toward nucleation temperature.
U.S. Pat. No. 3,721,732 issued to Knippenberg et al. on Mar. 20, 1973, contained contradictory advice concerning the addition of silicon to iron catalyst particles. In claim 4 as well as while discussing cubic growth, the patent speculated that admixing silicon to iron may facilitate cubic crystal growth. However, earlier in the patent text, it was stated that iron may without objection contain carbon and silicon or other elements, but improvement in the growth of crystals, as a result, had not been found. At best, this patent left doubt about whether silicon addition to iron catalyst particles would help in the overall growth of the silicon carbide whiskers. The role of specially treated catalyst particles in prompt initiation of nucleation at catalyst sites was not commented upon in this patent.
Overall, a need still existed for a method to avoid the delay in the growth of silicon carbide whiskers after reaching the growth temperature while the melted catalyst takes up from the surrounding furnance atmosphere the necessary percentages of carbon and silicon to initiate growth. When the catalyst particles were placed upon a carbon substrate, the particle interacted with the substrate in order to absorb carbon, which along with the accretion of silicon from the vapor results in an incubation time before growth can be initiated.